Cheese is a milk product that is generally rich in flavor and contains many high-quality nutrients. There are a great many varieties of cheese but all are produced in a similar manner. First, raw or pasteurized milk is cultured, clotted by acid, rennet or both. The resulting curd is then cut and shaped into the desired form with or without pressing. Fresh cheeses such as cottage cheese or cream cheese do not require any further processing. Other varieties of cheese are, however, subsequently cured or ripened to obtain a desired consistency, flavor and aroma. These characteristics are produced by a partial breakdown of milk proteins and fat by the action of microbial, milk and rennet enzymes.
The acid produced during the manufacture of cheese results from the fermentation of the milk sugar, lactose. Generally, this fermentation is initiated by the addition to the milk of a culture of specially selected acid bacteria. Known as a starter culture, many different types of lactic acid bacteria may be utilized for this purpose. The acid production in the cheese curd resulting from the activity of the starter culture advantageously functions to retard the growth of other bacteria that would otherwise cause undesirable fermentations in the cheese. Further, the acid production also favors the expulsion of the whey and the fusion of the curd particles.
The lactic acid starter cultures utilized in cheese making may comprise single or mixed strains of bacteria. All, however, must convert milk sugar in the curd into lactic acid within a reasonable time if a high quality cheese is to result. Several factors may, however, prevent this conversion. Of these factors, the most important is bacteriophage attack. Specifically, cheese production loss due to phage attacks on lactic culture is the number one problem faced by the dairy products industry today.
Bacteriophages or phages, are viruses that attack a lactic acid bacteria cell, commandeer the biosynthetic or reproductive machinery of the cell, produce new phages and in the process lyse the bacteria cell. Phages are prevalent in cheese making facilities. They may contaminate the milk in a number of ways including through use of infected starter cultures or through contact with phage-carrying dust particles.
Phages effectively slow down or totally inhibit the activity of the starter culture. As a result, the milk fermentation medium is often insufficiently soured and insufficient acid is produced to retard the growth of undesirable bacteria that cause undesired fermentation products. Hence, the cheese spoils resulting in an undesired consistency, flavor, and/or aroma.
It is known that phage multiplication is influenced by temperature, pH and calcium content of the medium among other factors. While phage outbreaks can be at least partially controlled by rigorous hygienic handling of starters, by culture rotation and/or by culturing starters in calcium-reduced media, further improvements in controlling phage proliferation and attacks on starter cultures are desired. Toward this end, however, it should be appreciated that attempts to isolate phage-resistant strains have generally not met with success. A need is therefore identified for an improved method of controlling bacteriophage attacks on lactic acid bacteria utilized in the cheese making process.